The superiority of GPS receiver technology over other competing technologies, in terms of the cost, reliability, mass, size, and power considerations, as proven in its various applications such as navigation, spacecraft orbit determination, and surveying, has constantly prompted the extension of GPS to other important areas that require increasingly more exacting performance from the GPS system. One such area, for example, is the application of GPS to the attitude determination of aircraft and earth orbiting satellites.
For the GPS receivers to provide the requisite precision, the existing error sources in the GPS system must be eliminated or greatly reduced. At the current state of GPS technology, the most significant error source is the signal multipath propagation. For example, in spacecraft operational environments, the GPS signal is reflected from various structural components of the spacecraft and these reflected components are received by the GPS receiver along with the desired direct line-of-sight (LOS) path. The reflected signals differ from the desired LOS path signal in terms of their delays, amplitudes, and phases. The carrier phase tracking loop provides no inherent discrimination against the multipath signals and thus tracks the phase of the composite signal corrupted by multipath components. The resulting differential carrier phase estimation error can be orders of magnitude higher compared to the case of no multipath propagation in many GPS applications. For example,, the measurements obtained by the RADCAL satellite GPS-ADS (Attitude Determination System) experiments have shown that the differential range error in such environment is of the order of 1 cm corresponding to an attitude determination error of about 0.5 degree. Thus for the GPS receivers to provide precision pointing knowledge (order of 1 arcmin or better with 1 meter antenna baseline) or a differential range accuracy of about 0.3 mm or better, the multipath effects must be suppressed by orders of magnitudes. Similar accuracy may also be desirable in other GPS precision applications in the presence of multipath signals such as GPS based geophysical measurements and precision surveying.
Among the past approaches to deal with the multipath problem, one approach involves reducing the early-late delay spacing among the correlators in the GPS receiver code lock loop. However while this approach reduces the code range errors to some extent, it does not aid in the carrier phase measurements accuracy that is the basis of most GPS precision applications. Moreover even the reduction in the code range error is limited and if the early-late spacing is smaller than the initial delay error due to multipath signals (easily the case with many multipath situations), then the loop error can be very high and the loop may not even track.
In the work of VanNee et al., a set of nonlinear implicit equations are derived on the basis of maximum likelihood estimation theory. These equations contain the amplitude,, phase and delay of multipath signals. The paper proposes to solve these highly nonlinear equations in a recursive manner, but there is no explicit solution presented. The method has been implemented in a Nov Atel receiver using multiple correlators. Although this approach is useful for code phase measurement and a benign multipath environment, it may not be suitable for severe multipath environment and for precision GPS applications based on carrier phase measurements.